Molecular Dissection Of Arabidopsis MPT Family In Regulation Of Abiotic Stress Tolerance

Plants are sessile organisms and hence they cannot escape unfavourable environmental conditions within their life cycle, such as high salinity, drought, waterlog, high or low temperature, pathogen attack and mechanical agitation. Among them, high salinity and drought are the main constraints in plant geographical distribution and crop productivity. Plants have evolved finely tuned high-salinity and osomotic signaling and resistance mechanisms. When exposed to salt or osomotic stress, plants respond rapidly and carry out protective processes, for example, physical adaptation (changes in cellular structure and organic functional adaptation), accumulation of osmoprotectants (soluble sugars, proline, betaine, etc.) in cytosol, increased contents of various antioxidants (superoxide dismutase, catalase, ascorbinase, etc.), phytohoromone (GA, ABA) regulated adaptatipon etc., which reprogram the biological activities and establish a new metabolism balance for stressed plants and help survive the salt or osomotic stress.Often associated with stress is a reduction in photosynthesis and/or respiration, which in turn results in energy deprivation and ultimately in growth arrest and cell death. Mitochondria can integrate numerous metabolic pathways that are important in adaptive responses to extreme environmental conditions, such as respiration and oxidative phosphorylation, the control of redox balance, and the metabolism of proline and ascorbate. The mitochondrial phosphate transporter/carrier (MPT/PiC) which is located in the mitochondrial inner membrane catalyzes phosphate transportation. Nevertheless, the relationship between MPT and environmental stress are still largely unknown.A total of three MPT genes have been cloned from the Arabidopsis genome, which were designed as AtMPT1 (AT2G17270), AtMPT2 (AT3G48850) and AtMPT3 (AT5G14040). Their expression modes and function are analyzed in detail:(1) The phylogenetic tree analysis showed that the evolution of MPTs family is conservertive in all specices. The data also indicate that AtMPT2 protein is more closely related with AtMPT3 than with AtMPT1.(2) By quantitative real-time reverse transcription-PCR (qRT-PCR) and promoter:β-glucouronidase (GUS) fusions, the expression patterns were different among three AtMPT genes during Arabidopsis development. Moreover, the expression level of AtMPT3 is much higher compared with the other two MPT genes, revealing that AtMPT3 plays a predominant role in Arabidopsis development.(3) Both the seed germination and seedling establishment of mpt mutants and OEMPTs were obviously inhibited by salt stress. Recent research proved that GAs contents had something to do with the adaptation to salt stress in Arabidopsis. And the levels of bioactive GAs were slightly reduced in WT, on the contrary evaluated in mpts mutants and OEMPTs after salt treatment compared to the control. Besides, the salt sensitivities of mpt mutants and OEMPTs were similar to WT after supplementing with UNI during the seeds germination and seedlings establishment stages. These results indicate that MPTs mediated early Arabidopsis responses to salt stress, most probably through bioactive GAs.(4) The expression of GAs metabolic-related genes were remarkably regulated by AtMPTs. The expressions of GA responsive and metabolic genes changed remarkably in mpt3 and OEMPT3 compared with WT plants under salt stress condition. In detail, the biosynthesis genes GA20ox1/2/3 and GA3ox1/2/4 in mpt3 and OEMPT3 were slightly higher than that in WT plants, whereas the induced transcriptional levels of most GAs-deactivation genes GA2oxs were lower than that in WT plants.(5) AtMPT3 was induced by osmotic stress. Both the seed germination and seedling establishment of mpt3 and OEMPT3 were obviously inhibited by osomotic stress. The levels of bioactive ABA were all elevated in WT, mpt3 mutants and OEMPT3 after osomotic treatment. However, the ABA concentration increased 2.09-fold in mpt3, 1.99-fold in OEMPT3 compared to untreated controls, only1.23-fold in WT. Moreover, the osomotic sensitivities of mpt3 and OEMPT3 were restored to normal after applicating with tungstate during the seeds germination and seedlings establishment stages. These results suggested that some increase in endogenous ABA levels during osomotic stress could contribute to the enhancement of stress sensitivity of the mpt3 and OEMPT3.(6) ABA metabolic genes changed remarkably in mpt3 and OEMPT3 compared with WT plants. The transcription levels of the genes involving in ABA metabolism were affected by variable transcription level of AtMPT3. In mpt3 mutant and OEMPT3, the transcriptional levels of ABA biosynthesis related genes NCEDs and AAOs was induced much higher than that of WT; moreover, the transcriptional level of ABA decompose genes CYP707As was induced much lower than that of WT, Which in turn leads to the increase of ABA levels in mpt3 and OEMPT3.(7) The relative GUS activity of the newly constructed P3 promoter showed that the induced levels of GhNHX1 promoter was largely enhanced by the repeated GT-1 box and the TM6s.